Typically, mobile communications antennas, such as those for use in the citizens band, are quarter wave-length, ground plane antennas. Size is the primary reason the quarter wave-length antenna is so prevalent, particularly for the citizens band frequency range. One-half wave-length in the citizens band frequency is approximately eighteen feet. It is quite clear that such an antenna is much too long for use as a mobile whip. Even the quarter wave-length antenna, which is approximately nine feet in length for the citizens band, is too long for most mobile installations, although some in fact do exist.
Most mobile antennas are electrically shortened, i.e., inductively loaded, quarter wave-length whips grounded to the vehicle to which they are attached. One reason for utilizing the ground plane quarter wave-length antena is that the feed point, which is a relatively low impedance point of the antenna, can be easily matched to the usual fifty ohm impedance of transmission lines. Since the quarter wave-length mobile ground plane antenna must be suitably grounded to the vehicle to which it is mounted, it requires some conductive surface to act as the ground plane, e.g., the body of the vehicle. Non-metallic automobile bodies and boats are typical examples of environments in which a ground plane is not readily available. Of course, this means that typical ground plane antennas may not readily be used in such environments.
There are a variety of techniques and devices for mounting ground plane antennas to vehicles. Antennas may be attached to a vehicle magnetically, by clips or clamps, or by drilling a hole through the surface of the body. The ground plane connections are usually conductive, although magnetic antennas are most often capacitively grounded to the surfaces on which they are magnetically retained. With magnetic mounts the cable must still pass through the car body, such as through a partially opened window, and cracking and breaking of the cable too frequently occurs. Many vehicles are not adaptable to the clip or clamp type mounts and therefore are limited to use of permanently mounted antennas, requiring a hole in the body, or to magnetic antennas. At the same time, many mobile radio operators are not satisfied with magnetic type antennas, and have no desire to punch or cut holes in their vehicle body.
It would be desirable, therefore, to have available a suitable mobile antenna which could be mounted to the surface of a non-conductive body or which could suitably be mounted to a vehicle to which other antennas are not adaptable and which is easily and rapidly mountable, while providing operating characteristics and radiation patterns which are equivalent to the usual inductively loaded quarter wave-length antennas.